The present invention relates to the communication of signals via optical fibers, particularly to tapered optical fibers and couplers. More particularly, the invention relates to wavelength selective tapered fiber gratings, add/drop filters and optical circuits for wavelength division multiplexed systems.
Low loss, wavelength selective filters are important components for optical fiber communication networks based on wavelength division multiplexing (WDM). WDM enables an individual optical fiber to transmit several or many channels simultaneously, at different spaced apart center wavelengths. As the art has developed, standardized wavelength band parameters have been set, for the number of channels, the total wavelength band, and the separation between channels. As a basic example, 30 nm may be devoted to a band of 4 channels having 10 nm separations between the center wavelengths.
Transmission of multiplexed signals without intervening wavelength signals or interference between them is complicated by the need for low loss both above and below the selected center wavelengths. In-fiber (e.g., erbium) amplifiers have a relatively wide window (e.g., 35 nm) and unwanted off-wavelength signals affect network reliability. Optical wave energy interacts with active and passive components which can have an adverse effect on insertion loss and signal attenuation, as well as introducing undesired backreflection. Devices such as fiber Bragg gratings are widely used in WDM systems. Most of such filters are used in a reflection mode, which provides higher wavelength selectivity than transmission mode, and write a Bragg grating in the core of the fiber (dopant in the core enables photosensitization and subsequent grating recording). Some of these filters, and also couplers made by fusing two fibers along a short length, are tapered to a reduced diameter waist. Whether tapered or not, the reflected wavelength, which is typically called a drop wavelength, is accompanied by signals of shorter wavelength that arise from lossy cladding modes.
In prior art fiber gratings, the grating periodicity written in the core has only limited transverse width, and the grating components couple light into lossy cladding modes which have only small wavelength separations from the reflected wavelength. The fact that the single mode optical fibers have low numerical aperture (ca 0.12) contributes to this condition. Heretofore the separation has been no greater than 10 nm, on the short wavelength side of the Bragg wavelength. This places a limitation on the placement of the signals and channels which can pass through a fiber grating. Tapered devices reduce the dopant core to a vestigial size and function, and so do not usefully employ Bragg gratings. As a consequence, index of refraction gratings have heretofore been recorded only in the core of the optical fiber, a volume of less than 1% of the total fiber. The presence of a taper dramatically reduces diffraction efficiency, typically to less than 1%, and the problems arising from the limited splitting of lossy modes from backreflected wavelengths still remain.
The splitting problem also exists in most fused couplers which are grating assisted, because light is largely confined in the original cores and overlaps only at the evanescent tails of the optical modes
It is now known, however, as described in U.S. Pat. No. 5,805,751 (assigned to the assignee of the present invention), that these disadvantages can be overcome in a fused coupler using novel concepts. A fused, small diameter, waist is formed by merging two elongated fibers along a short length. The fibers are tapered to the waist sufficiently slowly, in adiabatic fashion, for light to evolve into a single local supermode at the waist. In this merged waist the optical mode propagation characteristics are effectively those of a multimode silica core/air cladding waveguide, the original core having diminished to a non-functional diameter in terms of wave confinement. The index of refraction grating couples to propagated waves with much increased diffraction efficiency, and the numerical aperture of the waveguide is much higher. Also modal propagation constants can be maintained at higher differentials than conventional fused couplers, so the difference between backreflected and back coupled lossy cladding mode light can also be higher. Even here, however, relaxation of design requirements is desirable in terms of cost and yield.
Obtaining comparable splitting properties, along with high coupling strength, in a tapered fiber grating has remained a desirable goal for device and system designers. Because of the need to avoid cumulative insertion losses and the restraints imposed by the presence of lossy wavelengths in close relation to dropped wavelengths, system configurations can be simplified or made more efficient if these limitations can be overcome.
A xe2x80x9cwaveguidexe2x80x9d herein is an elongated structure comprised of an optical guiding region of relatively high refractive index transparent material (the core) surrounded by a material of lower refractive index (the cladding), the refractive indices being selected for transmitting an optical mode in the longitudinal direction.
An xe2x80x9coptical fiberxe2x80x9d herein is an elongated structure of nominally circular cross section comprised of a xe2x80x9ccorexe2x80x9d of relatively high refractive index material surrounded by a xe2x80x9ccladdingxe2x80x9d of lower refractive index material, adapted for transmitting an optical mode in the longitudinal direction.
An xe2x80x9cair-cladxe2x80x9d fiber is one in which the original core is too small to be effective and in which transmission is confined by the reduced original cladding and the surrounding environment (typically air).
A xe2x80x9clossy modexe2x80x9d is any of the higher order optical modes that exist within the waist region of the tapered fiber, wherein optical propagation is dictated by the air-glass waveguide. Power in these optical modes is lost as they propagate through the taper into the single mode core of the original fiber, hence they are lossy.
A xe2x80x9cgratingxe2x80x9d herein is a region wherein the refractive index varies as a function of distance in the medium. The variation typically, but not necessarily, is such that the distance between adjacent index maxima is constant.
A xe2x80x9ctapered doped-cladding fiber gratingxe2x80x9d is photosensitive cladding optical fiber that is elongated in a central region so that propagation characteristics in the central region are dictated by air-glass propagation and wherein a grating is present in the central region.
The xe2x80x9cbandwidthxe2x80x9d of a grating is the wavelength separation between those two points for which the reflectivity of grating is 50% of the peak reflectivity of the grating.
A xe2x80x9cwaistxe2x80x9d herein refers to that portion of an elongated structure with minimum circumference.
A xe2x80x9ctransversely symmetricxe2x80x9d grating is an index of refraction grating in which the index variation as a function of distance from the central axis of the waveguide along a direction perpendicular to the longitudinal axis is identical to the index variation in the opposite direction, perpendicular to the longitudinal axis. A transversely symmetric grating possesses grating vector components at a zero degree angle to the longitudinal axis or mode propagation direction of the waveguide. Orthogonal modes are not efficiently coupled by a transversely symmetric grating.
A xe2x80x9csupermodexe2x80x9d is the optical eigenmode of the complete, composite waveguide structure.
A high diffraction efficiency optical fiber device in accordance with the invention utilizes a grating distributed within a doped cladding portion of tapered section, to separate a desired optical wavelength from other wavelengths and from lossy modes as well. This enables a number of optical circuits to be developed which utilize such devices to advantage by nullifying some characteristics of other components.
More specifically, individual devices in accordance with the invention include a waist region of small diameter achieved by drawing down a conventional-size optical fiber to a very small diameter, such as less than 10 microns. The device incorporates a substantial radial distribution of dopant extending out from the central core, and a grating of selected periodicity, the refractivity variations in which are transversely distributed through the dopant volume, to be in excess of 20% of the cross-sectional fiber area at the waist. The periodicity is selected such that signals of a chosen wavelength are reflected (the grating being bidirectional) with high diffraction efficiency. At the same time, the diameter of the waist region is chosen to increase the difference (splitting) between the reflected (or drop) wavelength and lossy cladding mode wavelengths. Thus the latter are shorter than the center backreflected wavelength by greater than 10 nm and for sufficiently small waist diameters ( less than 1 um) they disappear completely.
Using the tapered doped cladding fiber grating with signals received from a source through a circulator, the chosen center wavelength is dropped to a detector back through the circulator, as other wavelengths outside the bandwidth of the filter pass through it to a different detector. In a specific example of such a device, the diameter of the waist region is in the range of 1-4 microns, the core-cladding index difference is approximately 0.45 and the dopant-rich cladding is in excess of about 50% of the cross-sectional area of the tapered waist.
In consequence of this improved splitting function system configurations are released from a number of design limitations. In-fiber amplifiers with wider bandwidths than the filters can be employed with greater freedom from wavelength restrictions. Other devices, such as wavelength selective couplers, can be designed to less strict tolerances if they are set to the same wavelength as the fiber grating and receive a signal from or send a signal to it. Systems in accordance with the invention may employ sequences of tapered doped cladding fiber gratings, each responsive to a different wavelength, in conjunction with sequences of wavelength responsive couplers set at the same wavelengths. By using the bidirectional reflective characteristics of the gratings and the reflective characteristics of the couplers, channels in a wavelength division multiplexed band can be added and dropped with lossy cladding modes in effect being nullified by the tapered fiber gratings. Adding and dropping of channels can be determined by the wavelengths that are present in the signal, and additionally by programming control using switches incorporated in the system.
In one example of an optical circuit, a bypass loop about a series of tapered doped cladding fiber gratings is formed by circulators at each end, forming the terminals of a bypass loop in which four port add/drop couplers are disposed that respond to wavelengths like those in the fiber gratings. Signals to be dropped reflect from the fiber gratings into the bypass loop, while signals to be add reflect off the couplers into a forward direction in the bypass loop, then via a circulator into the reverse transmission direction before being reflected as output off the appropriate fiber grating. This system is made programmable by the incorporation of switches in the bypass loop, together with shunt paths for the couplers.
In a different arrangement in accordance with the inventions, an optical signal router is provided by using a series of switches on a throughput line, and shunting successive pairs of switches with bidirectional tapered doped cladding fiber gratings. Circulators at each end of the switch series enable signals on the throughput line to be added or dropped at selected wavelengths in programmed fashion.
In other basic configurations, a circulator on a throughput line incorporating a series of tapered doped cladding fibers is coupled to another optical line which includes a number of add or drop couplers. Signals at wavelengths to be added to the throughput line are reflected to the circulator and back to the appropriate tapered fiber for redirection along the throughput line. Where signals are to be dropped they are reflected first off the appropriate tapered fiber and back through a circulator to the coupler sequence.